That Chipset is harder to find out . Guessing for good reason . I'm guessing Asus Strix B450-E will just get a nice luck of paint and will be B550 given an idea of what to expect. 12 phase B450 does seems a little overkill and made sure it's damn hard to get ! Carbon is a good board but if Strix E as available here... Would be flying off the shelves .
Question is... If B550 going to really limit PCIe to force x570 to look more appealing whilst leaving PCIe 4.0 to it's bigger brother.. don't see a need for it on the b550 .
Also question to whether it'll all be in-house , guessing most likely though to keep costs down .
This would mean X470 will still end up the better choice compared to B550.
If you describe it rightly, a B550 can be an actual A520.